C 指向 Swift 指针的指针

Question

我正在尝试使用指针将一段 C 代码转换为 Swift 3。这是 C 代码中的相关部分。

Float32 sampleArray[256] = { // Array is 256 Float values
0.00000, 0.00000, -0.00060, 0.00000, -0.00237, 0.00000, -0.00528 etc.etc}


float *p1, *p2, temp;
long i, bitm, j;

for (i = 2; i < 128-2; i += 2) {

    for (bitm = 2, j = 0; bitm < 256; bitm <<= 1) {
        if (i & bitm){j++;}
        j <<= 1;
    }

    if (i < j ) {
        p1 = sampleArray+i;
        p2 = sampleArray+j;
        temp = *p1;
        *(p1++) = *p2;    //Stuck from this point onwards
        *(p2++) = temp;
        temp = *p1;
        *p1 = *p2;
        *p2 = temp;
    }
}//eo for

这是我的快速尝试。显然使用相同类型的数组，但在 swift 中

var bitm:CLong
var j:CLong
var temp:Float
var p1:UnsafeMutablePointer<Float>
var p2:UnsafeMutablePointer<Float>

for i in stride(from: 2, to: 128-2, by: 2){
    j = 0
    bitm = 2;
    while bitm < 256 {
        if (i & bitm != 0){ j = j + 1}
        j = j<<1
        bitm<<=1
    }

    if (i < j){
        p1 = UnsafeMutablePointer(mutating: sampleArray)+i
        p2 = UnsafeMutablePointer(mutating: sampleArray)+j
        temp = p1.pointee
        /* looks ok up to this point*/

    }

}//eo stride

正如你所知道的，我已经做到了

*(p1++) = *p2;    

我认为这可能是使用

移动指针的情况

p1.advanced(by: 1)

我注意到最后四行的任何帮助将不胜感激

p2.pointee = somevalue

描述数组中需要处理的点。如果我将所有内容替换为

 sampleArray[j] = sampleArray[i] 

让我更接近于几行代码的工作。但我想保留它作为指针。这样我就可以了解它们是如何快速工作的。谢谢..

为了清楚起见，这里是 sampleArray

    var sampleArray:[Float] = [
        0.00000, 0.00000, -0.00060, 0.00000, -0.00237, 0.00000, -0.00528, 0.00000, -0.00931, 0.00000, -0.01440, 0.00000, -0.02052, 0.00000, -0.02762, 0.00000, -0.03566, 0.00000, -0.04460, 0.00000, -0.05443, 0.00000, -0.06502, 0.00000, -0.07634, 0.00000, -0.08836, 0.00000, -0.10101, 0.00000, -0.11424, 0.00000, -0.12799, 0.00000, -0.14238, 0.00000, -0.15703, 0.00000, -0.17203, 0.00000, -0.18733, 0.00000, -0.20286, 0.00000, -0.21858, 0.00000, -0.23470, 0.00000, -0.25063, 0.00000, -0.26657, 0.00000, -0.28247, 0.00000, -0.29827, 0.00000, -0.31392, 0.00000, -0.32979, 0.00000, -0.34501, 0.00000, -0.35993, 0.00000, -0.37450, 0.00000, -0.38868, 0.00000, -0.40242, 0.00000, -0.41569, 0.00000, -0.42904, 0.00000, -0.44126, 0.00000, -0.45289, 0.00000, -0.46390, 0.00000, -0.47426, 0.00000, -0.48394, 0.00000, -0.49365, 0.00000, -0.50193, 0.00000, -0.50945, 0.00000, -0.51621, 0.00000, -0.52219, 0.00000, -0.52738, 0.00000, -0.53176, 0.00000, -0.53621, 0.00000, -0.53900, 0.00000, -0.54096, 0.00000, -0.54212, 0.00000, -0.54246, 0.00000, -0.54199, 0.00000, -0.54169, 0.00000, -0.53965, 0.00000, -0.53684, 0.00000, -0.53327, 0.00000, -0.52895, 0.00000, -0.52392, 0.00000, -0.51918, 0.00000, -0.51276, 0.00000, -0.50570, 0.00000, -0.49800, 0.00000, -0.48971, 0.00000, -0.48084, 0.00000, -0.47144, 0.00000, -0.46251, 0.00000, -0.45212, 0.00000, -0.44129, 0.00000, -0.43005, 0.00000, -0.41844, 0.00000, -0.40650, 0.00000, -0.39520, 0.00000, -0.38269, 0.00000, -0.36996, 0.00000, -0.35704, 0.00000, -0.34396, 0.00000, -0.33078, 0.00000, -0.31752, 0.00000, -0.30506, 0.00000, -0.29174, 0.00000, -0.27845, 0.00000, -0.26523, 0.00000, -0.25210, 0.00000, -0.23910, 0.00000, -0.22698, 0.00000, -0.21432, 0.00000, -0.20187, 0.00000, -0.18967, 0.00000, -0.17775, 0.00000, -0.16612, 0.00000, -0.15538, 0.00000, -0.14439, 0.00000, -0.13376, 0.00000, -0.12350, 0.00000, -0.11364, 0.00000, -0.10418, 0.00000, -0.09514, 0.00000, -0.08693, 0.00000, -0.07873, 0.00000, -0.07097, 0.00000, -0.06366, 0.00000, -0.05679, 0.00000, -0.05038, 0.00000, -0.04467, 0.00000, -0.03911, 0.00000, -0.03400, 0.00000, -0.02931, 0.00000, -0.02504, 0.00000, -0.02118, 0.00000, -0.01772, 0.00000, -0.01477, 0.00000, -0.01203, 0.00000, -0.00964, 0.00000, -0.00758, 0.00000, -0.00583, 0.00000, -0.00437, 0.00000, -0.00322, 0.00000, -0.00225, 0.00000, -0.00149, 0.00000, -0.00093, 0.00000, -0.00052, 0.00000, -0.00026, 0.00000, -0.00011, 0.00000, -0.00003, 0.00000, 0.00000, 0.00000,]

这就是根据 C 代码的输出结果

0.0000, 0.0000, -0.4980, 0.0000, -0.3745, 0.0000, -0.1235, 0.0000, -0.1280, 0.0000, -0.3175, 0.0000, -0.5318, 0.0000, -0.0177, 0.0000, -0.0357, 0.0000, -0.4184, 0.0000, -0.4743, 0.0000, -0.0568, 0.0000, -0.2506, 0.0000, -0.2143, 0.0000, -0.5396, 0.0000, -0.0022, 0.0000, -0.0093, 0.0000, -0.4625, 0.0000, -0.4290, 0.0000, -0.0869, 0.0000, -0.1873, 0.0000, -0.2652, 0.0000, -0.5421, 0.0000, -0.0076, 0.0000, -0.0763, 0.0000, -0.3700, 0.0000, -0.5094, 0.0000, -0.0340, 0.0000, -0.3139, 0.0000, -0.1661, 0.0000, -0.5239, 0.0000, -0.0003, 0.0000, -0.0024, 0.0000, -0.4808, 0.0000, -0.4024, 0.0000, -0.1042, 0.0000, -0.1570, 0.0000, -0.2917, 0.0000, -0.5390, 0.0000, -0.0120, 0.0000, -0.0544, 0.0000, -0.3952, 0.0000, -0.4937, 0.0000, -0.0447, 0.0000, -0.2825, 0.0000, -0.1897, 0.0000, -0.5333, 0.0000, -0.0009, 0.0000, -0.0205, 0.0000, -0.4413, 0.0000, -0.4529, 0.0000, -0.0710, 0.0000, -0.2186, 0.0000, -0.2391, 0.0000, -0.5420, 0.0000, -0.0044, 0.0000, -0.1010, 0.0000, -0.3440, 0.0000, -0.5222, 0.0000, -0.0250, 0.0000, -0.3450, 0.0000, -0.1444, 0.0000, -0.5128, 0.0000, -0.5057, 0.0000, -0.0006, 0.0000, -0.4897, 0.0000, -0.3887, 0.0000, -0.4714, 0.0000, -0.1424, 0.0000, -0.4521, 0.0000, -0.5362, 0.0000, -0.4301, 0.0000, -0.0446, 0.0000, -0.4065, 0.0000, -0.4839, 0.0000, -0.3827, 0.0000, -0.2666, 0.0000, -0.3570, 0.0000, -0.5368, 0.0000, -0.3308, 0.0000, -0.0144, 0.0000, -0.3051, 0.0000, -0.4413, 0.0000, -0.2785, 0.0000, -0.2029, 0.0000, -0.2521, 0.0000, -0.5425, 0.0000, -0.2270, 0.0000, -0.0884, 0.0000, -0.2019, 0.0000, -0.5162, 0.0000, -0.1777, 0.0000, -0.3298, 0.0000, -0.1554, 0.0000, -0.5192, 0.0000, -0.1338, 0.0000, -0.0053, 0.0000, -0.1136, 0.0000, -0.4157, 0.0000, -0.0951, 0.0000, -0.1720, 0.0000, -0.0787, 0.0000, -0.5410, 0.0000, -0.0637, 0.0000, -0.0650, 0.0000, -0.0504, 0.0000, -0.5019, 0.0000, -0.0391, 0.0000, -0.2983, 0.0000, -0.0293, 0.0000, -0.5290, 0.0000, -0.0212, 0.0000, -0.0276, 0.0000, -0.0148, 0.0000, -0.4639, 0.0000, -0.0096, 0.0000, -0.2347, 0.0000, -0.0058, 0.0000, -0.5417, 0.0000, -0.0032, 0.0000, -0.1142, 0.0000, -0.0015, 0.0000, -0.5274, 0.0000, -0.0005, 0.0000, -0.3599, 0.0000, -0.0001, 0.0000, 0.0000, 0.0000, 0.0000, 0.0000,

Answer 1

您的代码中有一件非常糟糕的事情:

UnsafeMutablePointer(mutating: sampleArray)

检查 Pointer part的 Using book

Constant Pointers

...

The pointer passed to the function is guaranteed to be valid only for the duration of the function call. Don’t try to persist the pointer and access it after the function has returned.

因此，传递给 UnsafeMutablePointer.init(mutating:) 的指针在初始化程序调用后可能无效。当然，返回的指针也可能无效。

如果你想使用指向 Swift 数组的指针，你需要将你的 sampleArray 声明为 var 并使用 withUnsafeMutableBufferPointer(_:)保证在闭包内有效:

sampleArray.withUnsafeMutableBufferPointer {bufferPointer in
    let sampleArrayPointer = bufferPointer.baseAddress!
    for i in stride(from: 2, to: 128-2, by: 2) {
        var j: CLong = 0
        var bitm: CLong = 2
        while bitm < 256 {
            if i & bitm != 0 { j += 1 }
            j <<= 1
            bitm <<= 1
        }

        if i < j {
            // p1 = sampleArray+i;
            var p1 = sampleArrayPointer + i
            // p2 = sampleArray+j;
            var p2 = sampleArrayPointer + j
            // temp = *p1;
            var temp = p1.pointee
            // *(p1++) = *p2;
            p1.pointee = p2.pointee
            p1 += 1
            // *(p2++) = temp;
            p2.pointee = temp
            p2 += 1
            // temp = *p1;
            temp = p1.pointee
            // *p1 = *p2;
            p1.pointee = p2.pointee
            // *p2 = temp;
            p2.pointee = temp
        }
    }//eo stride}
}

但正如 Codo 的回答中所建议的，在这种情况下您无需使用指针。